Deploy software at public school

I'm already sorry that I have to ask this question, because there are many forum threads about this, but none of these seem to fit my requirements and i dont want to mess this up.

Here is my problem:
A friend of mine has asked me to develop some tiny to to categorize and sort some data.
Now I've written a Qt application and it's working pretty fine. The friend of mine is a teacher at a public school and wants to use the software there. Now he said that the principal want's to give me some money for the time I've spent in developing.

What do I have to take care of now in terms of licencing, terms and conditions and so on?
I don't want to do anything illegal!

The project is very specific and I don't think anyone else would want to use it. I also don't want to sell it online or sth.

If possible you can upload it to say https://github.com
Then its fully open source and you need not worry about anything else then to tell
"tax" about the income ;)

That said, it's also possible to just make a deployment folder and just give him a binary version.
As long as you dynamic link to Qt ( and only use widgets covered by it) then you can keep source close
and still give your friend a version.

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If you are using the LGPL version of Qt, you don't need to provide your code. However you have to have a clear place where you specify that your software is built using Qt. e.g. the "About" box of your application. Note that this is not Qt specific, most free and OpenSource project have a license that specify this requirement.

If you are dynamically linking to Qt, then LGPL is fine. If you would like to link against the static version of Qt, then it gets more complicated as you need to provide the means to rebuild your application with a different version of Qt. So basically, for your small tool, just go the dynamic way, the deployment's been greatly simplified with the years.

If you distribute the binaries with Qt binaries included you have to provide the Qt source code, too (or the written offer; see the licence for details). On the other hand, if it's going to be in-house product only it's possible (but not certain!) that it's not counted as public redistributing. If an employee writes code for a company, the company, not the employee, owns the copyright. And the company doesn't have to distribute the software outside and therefore isn't bind to the redistributing obligations of LGPL. In that case they have to prohibit distributing somehow so that employees don't do it on purpose or accidentally (if they do, the company must fullfill the obligations).

In this case I wouldn't worry about it - legal is legal and I want to obey the law and high moral standards, but sometimes you just have to be realistic and practical. If you want to be sure, ask the Qt Company legal department, not here.

In any case you can take money if you want to, it's not related to the these licencing issues and obligations as long as you don't try to sell Qt.